1. Field of the Invention
The invention is directed to photochemically curable, millable polyurethane gums, and to methods for the preparation and for the use thereof.
2. Background of the Invention
Early attempts to replace natural rubber latex, e.g. that derived from Hevea brasiliensis, with synthetic rubbers were problematic, and numerous synthetic rubbers such as those based on butadiene and styrene were initially unsuccessful. Natural rubber consists predominately of naturally polymerized isoprene units, and the physical and chemical properties of natural rubber, coupled with the use and modification of natural latex dating back to aboriginal times, has resulted in a wide variety of uses of natural rubber, in both the vulcanized and unvulcanized states. However, natural rubber is deficient in numerous applications, such as those requiring greater hardness and abrasion resistance. It is for many of these applications that polymers have been sought which can modify the rubber properties or replace natural rubber in its entirely. Millable polyurethane rubber has been a successful modifier and replacement.
Millable polyurethane rubber is distinctly different from other types of polyurethanes. The first polyurethanes were invented by Dr. Otto Bayer in 1937, and since that time, have experienced enormous growth in both technology and applications. For example, slab polyurethane foam is produced in enormous quantities throughout the world for use, for example, in seating products. In the automotive industry, molded polyurethane foams are used for the same purposes. Thermoplastic polyurethane, or “TPU” has been developed for use in many products, one of which include the jounce bumpers in modern automotive suspension systems. Spandex™ elastomers have a wide range of uses, including stretch fabrics in the clothing industry. In the 1950s, millable polyurethane rubbers were touted especially for use in producing tires for vehicles. Early players in this latter market included Bayer, U.S. Rubber (which later became Uniroyal) and DuPont. In the 1960s, research and production expanded to other companies such as General Tire, Goodyear, Michelin, and American Cyanamid, as well as Witco and Thiokol.
Polyurethanes are produced by the reaction of a di- or polyisocyanate with a glycol or hydroxyl-functional polyol, or both. The hydroxyl-functional polyol may be a polyether polyol or a polyester polyol. Millable polyurethane rubber is different from other polyurethanes which are either stoichiometricly indexed or over-indexed. In order to provide a millable gum polymer product, millable polyurethane polymers are under-indexed, i.e. contain an excess of hydroxyl functionality as compared with the amount of isocyanate functionality. Preparation of millable polyurethane polymers, or “gums” proved to be difficult, resulting in very inconsistent materials. Largely for this reason, the majority of manufacturers ceased production of millable polyurethanes in the 1970s. However, millable polyurethane still constitutes an important area of technology due to the chemical and physical properties which can be obtained upon vulcanization of the millable polyurethanes. Vulcanization can be divided into two categories: peroxide cure and sulfur cure.
In peroxide cured millable polyurethanes, an organic peroxide is milled with the polyurethane rubber at a temperature where the organic peroxide is substantially stable against decomposition. Following milling of the rubber, peroxide, and other ingredients such as antioxidants, fillers, pigments, etc., the milled rubber product is molded into the shape of the final article and cured at elevated temperature. At the elevated temperature cure, the organic peroxide decomposes to form free radicals which initiate cross-linking. In sulfur cured millable polyurethanes, sulfur or sulfur compounds such as thiuram disulfide and other ingredients are milled into the polyurethane rubber. Following molding, vulcanization then occurs at elevated temperature, by much the same process which has been used to produce vulcanized rubber since the introduction of vulcanization by Charles Goodyear many years ago.
The difficulty of producing millable polyurethane rubber within specification in a consistent manner has largely been overcome by the two remaining principal producers of millable polyurethane rubber: Rhein Chemie, and TSE industries, the latter being the world's largest producer of millable polyurethane rubber. However, the production of articles from millable polyurethanes has remained problematic, in part due to the tackiness and low physical strength of molded articles produced from the millable gums prior to cure. In addition, the necessity to carefully monitor the temperature during milling, the cost of the ovens necessary for the high temperature cure or “vulcanization”, and the time which vulcanization requires, has also limited applications of millable polyurethane rubber. This is especially the case with continuously produced molded profiles, which have been impossible to produce, despite a long felt need for such products and the maturation of the technology over the decades.
Thus, there has been a long felt need to provide a millable polyurethane rubber gum material which can be more easily compounded, which can avoid totally or at least mitigate increased production time and expense related to high temperature vulcanization, and can be used to produce molded profiles by a continuous process. Yet despite decades of research directed to improvements in these factors, these goals have not been met.